Latches are used in electric circuits to hold information temporarily. For example, in a computer system, incoming information may pass through a latch where it is held for a certain time so that it can be read.
The development of sub-micron CMOS technologies results in new standards for voltage supplies. Currently, CMOS circuits use voltage supplies as low as 3.3V, and in the future, voltage supplies as low as 2.5V will probably be used. A traditional ECL gate typically uses voltages of approximately 4.5V-5V. Therefore, there is a need for circuits that can operate at lower voltages. Also the technical development increases the need for faster circuits than those available today.
Latches can be designed to work either in a differential mode or in a single-ended mode.
In single-ended mode, the voltage at one of the inputs is held constant while the other one varies between two modes, one of which is higher, the other lower than the constant voltage at the first input. One drawback with the single-ended mode is that it requires at least twice the voltage swing of the differential mode. Typically the voltage swing in single-ended mode is 600 mV-800 mV.
In differential mode the voltage at both input terminals are varied, and a voltage swing of 200 mV-300 mV is required. This configuration is faster and also less sensitive to noise than the single-ended mode.
In logical circuits, several levels of transistors may be used, so that the output of one transistor controls a transistor at the next level. In ECL logic 3-4 levels of transistors are typically used. By reducing the number of levels, the required supply voltage is reduced. With a supply voltage of 2.5V, only one level of transistors can be used, together with one resistor and one current source.
In Razavi et al. "Design Techniques for Low-Voltage High-Speed Digital Bipolar Circuits", IEEE Journal of Solid-State Circuits, Vol. 29, No. 3, March 1994, a number of low-voltage circuits based on the ECL circuits are disclosed, among them a D-latch. This latch operates in a single ended mode and requires a supply voltage of 2.5V and a voltage swing of approximately 600 mV-800 mV.